When a living NPC is knocked out, they will become unconcious bodies if picked up. Bodies will alert hostiles if spotted. Not being doctors, they will almost always think a knocked out person is dead. If thrown far enough, damaged with weapons or thrown in water (or on fire), the NPC will die. They will make the same moans and grunts as if they were alive.
Carrying unconcious bodies or corpses will considerably slow Garrett down. He can still jump and swim, though swimming will kill an unconcious NPC. Even though the body is over his right shoulder supported by his arm, he can still climb ladders, and even ropes remarkably fast for being heavy and having use of only one arm.
NPCs will often have shocked or flustered reactions if he is spotted carrying a body around.